Subterranean reservoirs of aqueous geothermal fluids --such as steam and/or hot brine--exist in many regions of the world. Such geothermal fluid reservoirs can contain vast amounts of thermal energy.
Many of the serious problems associated with the production and use of geothermal fluids can be attributed to the unusually complex chemical composition of these geothermal fluids. The aquifer conditions also tend to leach large amounts of salts, minerals, and other elements from the formations into the geothermal fluids. The concentration of these elements leached into the geothermal fluids tends to reach saturation levels. This tendency results from the extended periods of time the hot geothermal fluids have been in contact with the materials of the formation. The acid gases (e.g., CO.sub.2 and H.sub.2 S) typically present may also help to dissolve even more contaminants in these geothermal fluids.
To generate power in a geothermal power plant from geothermal brines at a reservoir temperature of over about 400.degree. F. and pressures above 400 psig, brines are typically flashed to a reduced pressure. Flashing converts some of the liquid water constituent of the brine into steam. The steam has been used in generally conventional steam. turbine-type power generators to generate electricity, e.g., in New Zealand. Steam used for power production typically contains volatile or other contaminants, e.g., brine carry-over, H.sub.2 S, and ammonia.
Whether the steam is derived directly from the geothermal resource or from flashed brine, after discharge from the power plant turbine, the contaminated steam is typically condensed in a water cooled surface condenser supplied by an evaporative cooling tower. A direct contact condenser may be used if feasible, e.g., if contamination of the cooling water can be accepted. Liquid condensate and a non-condensable gas (at condenser conditions) are the effluents typically produced in the surface condenser.
The volatile contaminants of the steam are, after condensation, partitioned between the condensate and non-condensable gas (NCG) streams. The portions of the volatile contaminants partitioned into the condensate and gas streams depend upon the geometry of the condenser and fluid property (e.g., temperature and pressure) conditions within the condenser.
The partitioning and other operating conditions cause the compositions of the NCG and condensate streams from these geothermal fluids to contain variable amounts of contaminants. The contaminants may cause both the condensate and NCG streams from the condenser to be corrosive, and in certain cases, they also may be toxic. The corrosive and toxic contaminants tend to make thence condenser effluent liquids and gases commercially unusable outside of the power plant.
The contaminated condensate stream is typically used in the power plant's cooling tower as a makeup to compensate for the evaporation loss of coolant water. This evaporation causes dissolved solids in the cooling water to be concentrated, and blowdown by discharge of the cooling water is required to remove dissolved solids, thus prevent undesired precipitation of solids in the cooling tower system.
The blowdown stream is most commonly disposed of by injection into a geothermal formation or by discharge to surface water (possibly after treatment to comply with environmental discharge regulations). Injection avoids treatment of the blowdown stream or contamination of surface waters and may also replenish the geothermal aquifer from which the geothermal fluid was produced. Injection may be especially important to avoid treatment costs when larger amounts of contaminated blowdown are involved.
The atmospheric disposal of the NCG stream produced in the geothermal power process may also present problems. Although CO.sub.2 is typically the primary constituent of these gases, ammonia and hydrogen sulfide are also common constituents. These constituents may present environmental, corrosion, and other handling problems. Prior art treatment of these constituents requires removal of these constituents prior to discharge to the atmosphere.
In addition to non-condensable gases (NCG) separated during the flash process, corrosion and emission of gases from cooling tower waters (when the cooling tower makeup fluid source is contaminated condensate) are further problems. Gases emitted from the cooling tower can include H.sub.2 S, NH.sub.3, SO.sub.2, and NO.sub.2. The NO.sub.2 may be formed by the oxidation of ammonia in the cooling water by naturally occurring bacteria. SO.sub.2 may also be generated by microorganisms, and the dissolved gases can be corrosive. Nitrites can also be formed and produce acids corrosive to the cooling tower system and injection well disposal piping.
To overcome these and other problems in cooling tower systems, inhibitors or other additives are commonly added to the cooling water. Heavy metal inhibitors have been effective in other cooling tower applications for controlling the growth of organisms and the emissions they cause, since these inhibitors are toxic to the organisms. However, multi-function, heavy metal inhibitors tend to form heavy metal sludges which are now classified as a toxic or hazardous waste material in many localities. Consequently, the disposal of these sludges is difficult and expensive.
Non-heavy metal (corrosion) inhibitors, such as phosphate-types, have recently been used in some condensate handling systems. These phosphate materials do not tend to form hazardous waste materials in the presence of ammonia, carbon dioxide, and/or hydrogen sulfide. However, unlike their counterpart heavy metal inhibitors, phosphate-type inhibitors have not been as effective in inhibiting corrosion, microorganism growth, and emissions.
In summary, many effluent streams from a geothermal power plant need to be treated, specifically including 1) contaminated condensate, 2) cooling tower blowdown, 3) NCG, and 4) vapor emissions from a cooling tower. Indeed, the treatment of the H.sub.2 S constituent of NCG has required significant capital and operating investments in many prior art geothermal power plants.